A. graham Bell speaks volumes in his book Forced Induction Performance Tuning about using blow-off valves to regulate the boost pressure of turbochargers. It's a concept that has fallen out of fashion in favour of the modern wastegate (which, admittedly, works similar to a BOV, except on the turbine side of the turbocharger). As the science of high-temperature materials advanced, the wastegate became more reliable, and hence more popular, since it is (logically) preferable to blow-off excess exhaust gas pressure through a wastegate (and hence regulate the turbine speed) than it is to run the turbine excessively fast and regulate the boost pressure by simply opening up a leak at the compressor outlet.
However, in the case of a supercharger there is (self evidently) no mechanism (such as a wastegate) to regulate the speed of the compressor wheel. The speed of the compressor is, quite obviously, a direct function of the engine speed in the case of a supercharger.
I therefore think that whoever tuned your car had set up your BOV to blow-off around 0.9bar. Short of your BOV having a leak (and this is not altogether uncommon) this is the only possible explanation. This would explain, most definitely, why you experienced problems when you removed the BOV! Either the boost climbed high enough (with the BOV now removed) to result in ignition problems, or your Go-Tech was detecting detonation and responding accordingly (possibly even cutting your ignition).
Whichever was occurring, I can however tell you this: If you are running your M20B25 to no greater than 0.9 bar boost, then you are flowing no more than 0.25 kg/s of air into your engine at 6500 rpm, and that will account for no more than 290 hp at your flywheel - and that's a fact.
Now, you mentioned a new BOV. Your BOV should be vacuum operated (ie, you have to suck on the diaphragm to make it open) and should be connected to the inlet manifold (ie: after the throttle valve). This way, when you take your foot off the throttle pedal, and the pressure inside the manifold falls lower than that at the outlet from the compressor (where the BOV should be located) the BOV will open and release pressure trapped between the compressor and the throttle valve. This is essential to prevent your compessor from going into what is called surge (where the airflow around the compressor blades becomes turbulent) due to the fact that the airflow through the compressor (due to the fact that the throttle is closed) is too low to sustain the the boost pressure that would otherwise be trapped between the compressor and the throttle.
I have a sneaky suspicion that your BOV did not have the one side of the diaphragm connected to the manifold. Hence, as the pressure built at the outlet of the compressor, the BOV progressively opened and leaked
What a waste! This would have certainly kept your boost pressure down, and would certainly keep your engine away from detonation and ignition problems.
excellent idea. Don't be surprised if this catches on with others. Some credit to Rix313 though I think for the effort he put in on his four-pot motors (thanks Rich for the info you PM'd me).
[/img]
